The present invention relates to the field of fluid handling devices and more particularly to an improved fluid mixing device. Still more particularly the fluid mixing device may be adapted for use in mixing liquids used in automated clinical chemistry analyzers.
A common requirement in fluid handling systems is the mixing of two fluid flows to form a third fluid flow. For example, automated clinical chemistry analyzers frequently require two fluid flows to be mixed together to form a third fluid flow that is then analyzed. A first fluid may be, for example, a patient sample such as serum, plasma, urine or spinal fluid (CSF). The second fluid may be a buffer which, when combined with the first fluid, controls primarily the pH, ionic strength and surfactant properties of the resulting mixture.
One such system requiring the combination of two fluid flows is the SYNCHRON CX.RTM.3 automated clinical chemistry analyzer which is commercially available from Beckman Instruments, Inc. (Brea, Calif. 92621). In this system, a probe carrying the patient sample is aligned above a sample injection cell. The probe is lowered into the cell with the tip of the probe coming to rest within a mixing chamber. The sample is pumped from the probe into the mixing chamber while a buffer solution is pumped through a separate conduit into the mixing chamber. The resulting mixture flows from the mixing chamber through an exit conduit to an electrolyte measuring flow cell to measure sodium, potassium, chloride and CO.sub.2. An essentially identical system is disclosed in U.S. Pat. No. 4,888,998, issued Dec. 26, 1989, which is incorporated herein by reference.
A cutaway side view of the mixing chamber in such commercially available system is illustrated in FIG. 5. As seen with reference to FIG. 5, a mixing chamber 100 includes a vertical conduit 108 which is adapted to receive a probe 102 having a tip 104. A conduit 106 is "T"-ed into the side of and is perpendicular to the conduit 108. The conduit 106 is connected to a source of buffer which is to mix with sample ejected through the probe tip 104. An exit conduit 110 forms a right angle with the conduit 108 to draw the combined sample and buffer from the mixing chamber 100.
Unfortunately, the "T" configuration of the prior art system may impede mixing of the sample and buffer for several reasons. As the sample flow from the tip 104 meets the buffer flow from the conduit 106, the flows may simply combine without mixing, resulting in laminar, separate flows within the conduits 108 and 110. The degree of laminar, separate flow is influenced by the vertical position of the tip 104 within the conduit 108, thus making the system sensitive to routine changes in probe tip position that occur, for example, due to normal wear and tear and routine replacement of the probe 102. Further, an air bubble may be trapped directly below the tip 104 within the flow of sample from the tip 104. Such an air bubble vibrates rapidly within the conduit 108, resulting in pulses or bursts of sample within the flow of buffer. Also, air trapped above the conduit 106, while not causing pulses or bursts in the combined flow, does gradually break up, flowing to the flow cell where such disbursed air can collect, adversely affecting measurements.
One result of these limitations and drawbacks may be inconsistent electrolyte measurements and adversely affected average precision. Thus, there is a need for improved fluid mixing to improve and stabilize the performance of the measurement system.
The improved fluid mixing device of the present invention overcomes the limitations noted in the prior device. The improved fluid mixing device may be formed directly in the sample injection cell and, more particularly, may replace the mixing chamber found in the prior art sample injection cell. In accordance with the present invention, the improved fluid mixing device includes a mixing chamber having a cylindrical side wall, end walls and a major axis parallel to and coaxial with the cylindrical side wall. A first fluid conduit joins the mixing chamber at a first fluid port formed in one of the end walls. A second fluid conduit joins the mixing chamber at a second fluid port, the second port being formed in the cylindrical side wall. The second fluid conduit and second fluid port are offset with respect to the major axis to direct a fluid flow from the second conduit through the second port generally along the side wall and around the major axis of the mixing chamber, creating a swirling action within the mixing chamber. A third fluid conduit joins the mixing chamber at a third port in the other end and serves as an exit conduit for the fluids mixed in the mixing chamber.
In the embodiment of the invention disclosed herein, the mixing chamber may form a first stage of a mixing device or configuration. A second stage includes positioning the third port off-center in the second end and generally aligning the third fluid conduit with the major axis. Yet a third stage of the mixing device or configuration disclosed herein may include a fourth conduit intersecting the third conduit. Center lines of the third and fourth conduits are offset and do not intersect.
In as preferred form of the invention, the second conduit is offset at less than a tangent with respect to the mixing chamber. This causes fluid flow to be directed from the second conduit through the second port partially toward the internally extended portion of the first fluid conduit to thereby create a turbulent swirling action.
In overall effect, the improved fluid mixing device of the present invention thoroughly and completely mixes the two streams of inlet fluids. When used in the sample injection cell of the automated clinical chemistry system described above, the improved mixing results in more consistent performance and better average precision in the measurement of electrolytes.